The crimping or waviness of collagen in biological tissues can be measured quantitatively in terms of amplitude and periodicity by means of polarized light microscopy and electron microscopy. In porcine aortic valves prepared for use as bioprosthetic cardiac valves, this crimping was shown to be influenced by the conditions of tissue fixation with low concentrations of glutaraldehyde: when fixation was used with no pressure applied, the crimping was similar to that in the native aortic valve; however, low pressure fixation caused marked reduction in the crimping of much of the collagen, and high pressure fixation almost eliminated it completely in the cords and the mid-cusp region. A technique has been developed for making these observations on intact aortic leaflets.